The present invention relates generally to a process and apparatus for the treatment of water using membrane technology to separate the constituents of a feed liquid, and specifically to a process and apparatus for purification and blending of an aqueous solution utilizing one or more first membrane process units having a first permeability connected in series with one or more second membrane-process units having a second permeability to produce a desired blended product water thus modified to improve its taste characteristics.
The following expressions are defined to provide a better understanding of the present specification and claims.
Thin film composite membrane (TFC) process: Purification of liquid occurs through a thin film composite membrane where a feed liquid is separated into retentate and a permeate.
Retentate: The portion of a feed liquid which is retained by a membrane.
Permeate: The portion of a feed liquid which passes through a membrane.
Nanofiltration (NF): Filtration of a liquid through a NF-membrane. Such membranes are usually ion-selective and reject ions at varying percentages depending on the total molarity of the feed liquid and the chemistry of the membrane. Further, such filtration membranes typically have a pore size of 0.0007 "PHgr"m to 0.007 "PHgr"m, such pore sizes corresponding to a molecular cutoff value ranging from 140 xc3x85 to 15,000 xc3x85.
Reverse osmosis (RO): A liquid separation process in which a solution is put through a semi-permeable membrane that excludes most soluble ions thus concentrating undesired soluble and insoluble species in the retentate. Such membranes allow the passage of some ions in the size range of 0 "PHgr"m to 0.0015 "PHgr"m, which corresponds to a molecular cutoff value of between 0 xc3x85 and 300 xc3x85. RO separation of solution where water is the solvent typically results in a permeate consisting of only water with most ions being exluded.
Permeable solutes: Substances dispersed in the liquid which are capable of passing through the membrane employed. Such substances may be electrolytes, nonelectrolytes, colloidal dispersions, and/or particles.
Feed liquid: Liquid fed to the first of a series of membrane process units. Such a liquid could be a solution wherein the solutes may be dissolved and/or undissolved. Undissolved material may need to be filtered out before the membrane processes.
Hardness: The combined concentrations of magnesium and calcium salts as measured by CaCO3.
Alkalinity: The measure of carbonate, bicarbonate, and hydroxide as CaCO3.
When a feed liquid is fed into a membrane process unit, the feed liquid is separated into a retentate and a permeate. The permeate is the portion of the feed liquid which passes through a membrane contained within the process unit. Contained within the permeate are permeable solutes. Those solutes which are not capable of passing through the membrane being employed, or non-permeable solutes, are retained within the retentate.
Whether a solute is permeable or non-permeable depends upon the type of membrane used. Certain membranes may be employed which exclude solutes based upon molecular size. As the permeability or pore size increases, so does the passage of the permeable solutes. In addition, an ion selective membrane may be employed which excludes a solute based upon its charge.
Separation by the reverse osmosis process, and more particularly, purification of water by reverse osmosis in order to produce drinkable or potable water, is known. U.S. Pat. No. 5,238,574 to Kawashima et al. describes an apparatus for treating salt water using a plurality of reverse osmosis membrane units. However, a disadvantage of these types of membrane systems is that they produce product water which is of such a high quality that its taste characteristics may be unacceptable. These unacceptable taste characteristics result because the permeates produced by such reverse osmosis membrane systems tend to have low alkalinity levels, and significantly reduced levels of dissolved salts. As a result, the product water has an off taste or a xe2x80x9cflatxe2x80x9d taste.
The poor taste characteristics of product water produced by reverse osmosis can be overcome by two known post-filtration blending methods. The first method employs blending a portion of the feed liquid or untreated water with the permeate to produce product water having desired levels of alkalinity and dissolved salts. A disadvantage of this method is that it requires constant adjustment of the volume of untreated water needed for blending. The constant adjustment of the volume of feed water is due to the constantly changing chemical characteristics and pressure of the feed water. Also, introduction of untreated water back into the product water reduces the attainable benefits of the membrane process in removing unwanted substances from the feed water.
The second method employs the addition of minerals to the permeate to achieve the desired taste characteristics. This latter method is undesirable due to its high cost and inefficiency, since the membrane process removed the existing minerals in the first place.
Therefore, it is a first object of the present invention to provide an improved process and apparatus for treating a feed liquid to produce a product liquid having certain desired physical properties, resulting in improved taste.
A second object of the present invention is to provide an improved process and apparatus for treating a feed liquid without the necessity of blending a portion of the feed liquid with the permeate in order to produce a product liquid having certain desired physical properties.
It is a further object of the present invention to provide an improved process and apparatus for treating drinking water by membrane processes without the necessity of introducing minerals to the permeate to produce a product liquid having desired taste properties.
It is another object of the present invention to provide an improved process and apparatus for treating drinking water using membrane processes having different permeablities or porosities which can be arranged to produce a blended product liquid having certain desired chemical characteristics regardless of the chemical properties of the feed liquid.
The above-listed objects are met or exceeded by the present process and apparatus for membrane processing of a feed liquid to produce a product liquid. In the present system, at least two distinct membrane types are employed, each type having a distinct permeability. Permeates of the two types are blended to produce a treated product liquid having the desired water quality. Preferably, the permeability of the first type of membrane unit has a higher solid content than the second type, where the retentate of the first type is the feed liquid for the second type.
More specifically, to produce the product liquid, a feed liquid, typically an aqueous solution such as tap water, is first fed to a primary membrane type. The primary membrane separates the feed liquid into a primary membrane retentate which is retained by the primary membrane, and a primary membrane permeate. The primary membrane retentate is then fed to a secondary membrane type. The secondary membrane separates the primary membrane retentate into a secondary membrane retentate which is retained by the secondary membrane, and a secondary membrane permeate. The final product liquid is formed by blending the primary membrane permeate with the secondary membrane permeate.
In the preferred embodiment, the primary membrane produces water of poorer quality than the secondary membrane. This is because the permeable solute concentration of the primary membrane permeate is greater than the permeable solute concentration of the secondary membrane permeate. Thus, the final product liquid is a blend of the products of two different levels of membrane treatment. In this manner, the taste of product drinking water can be adjusted without introducing untreated water, and without adding supplemental minerals.
Alternate embodiments include the addition of one or more supplemental primary membranes connected in series with the primary membrane, and also include one or more supplemental secondary membranes connected in series with the secondary membrane. In this case, the primary membrane retentate is fed to a first supplemental primary membrane, and the retentate formed by the first supplemental primary membrane is fed to a second supplemental primary membrane. The retentate formed by the last supplemental primary membrane is then fed to the secondary membrane as feed liquid. The secondary membrane retentate formed by the secondary membrane may either be collected and disposed of, or may be fed to one or more supplemental secondary membranes. If the secondary membrane retentate is fed to a supplemental secondary membrane, the secondary membrane retentate (or retentate from a prior supplemental secondary membrane) is separated into a supplemental secondary membrane retentate and supplemental secondary membrane permeate.
The product water is formed by blending the primary membrane permeate and all permeates formed by the supplemental primary membranes with the secondary membrane permeate and all permeates formed by the supplemental primary membranes.